rash between hind legs

Our 2 year old lab has a skin condition between his hind legs and his belly.  It doesn't itch; he doesn't lick it.  Some are red bumps and some  look like pimples with whiteheads. It is not a heavy rash; each bump/pimple is surrounded by healthy skin without any irritation. Approximately 20 pimples are visible, about 5 of those have the whiteheads.

We know he has pollen allergies that make his skin itchy; he had a cortisone shot and now he seems fine.
The skin condition is new.

[el_em_en_oh]

Could they be ant bites?

Not sure if you get fire-ants up there in Twin Lakes, MI, but down here in Texas (), our dogs get into ants all the time, and what you've described sounds precisely like what they get.

[Jaybay]

The pimples concern me.  Whatever the underlying cause of the problem, it sounds like he now has a secondary skin infection.  I also agree with el_em above that fire ants are a possibility but don't think they've marched as far north as Michigan yet.  Their stings result in localized pockets of infection just like pimples.  

Another possibility is mange.  Did your vet do a skin scraping for that when your dog got the cortisone shot?  He could still have mange mites chewing away on him but they aren't itching because of the cortisone.

[Misfits4Me]

Usually, Pimple like bumps filled with pus is a Staph Infection.....Antibiotics are needed to clear it up..............